Magnetic random access memories (MRAMs) have been the object of a renewed interest with the discovery of magnetic tunnel junctions (MTJ) having a strong magnetoresistance at ambient temperature. These MRAMs present many advantages such as speed (a few nanoseconds of duration of writing and reading), nonvolatility, and insensitivity to ionizing radiation. Consequently, they are increasingly replacing memory that uses more conventional technology based on the charge state of a capacitor (DRAM, SRAM, FLASH).
In conventional MTJ based MRAM, the memory cell includes a magnetic tunnel junction that comprises a stack of several alternatively magnetic and non-magnetic metallic layers. Examples of conventional MTJ-based MRAM devices are described in U.S. Pat. No. 5,640,343. In their simplest forms, the magnetic tunnel junctions of MTJ-based MRAM are formed from two magnetic layers of different coercivity that are separated by an insulating thin layer. The first layer (or reference layer) of the magnetic tunnel junction is characterized by a fixed magnetization; whereas, the second layer (or storage layer) is characterized by a magnetization direction that can be changed. When the respective magnetizations of the reference layers and the storage layer are antiparallel, the resistance of the magnetic tunnel junction is high. On the other hand, when the respective magnetizations are parallel, the resistance of the magnetic tunnel junction becomes low.
Preferentially, the reference layer and the storage layer are made of 3d metals such as Fe, Co or Ni or their alloys. Eventually, boron can be added in the layer composition in order obtain an amorphous morphology and a flat interface. The insulating layer typically comprises alumina (Al2O3) or magnesium oxide (MgO). Preferentially, the reference layer itself can be formed from several layers as described, for instance, in U.S. Pat. No. 5,583,725, in order to form a synthetic antiferromagnetic layer. A double tunnel junction as described in the paper by Y. Saito et al., Journal of Magnetism and Magnetic Materials Vol. 223 (2001), p. 293, can also be used. In this case, the storage layer is sandwiched between two thin insulating layers with respective reference layers located on the opposite sides of the thin insulating layers.
FIG. 1 shows a memory cell 1 of a conventional MTJ-based MRAM. The memory cell 1 includes a magnetic tunnel junction 2 that comprises a storage layer 21, an insulating layer 22 and a reference layer 23. The magnetic tunnel junction 2 is illustrated as being disposed between a selection CMOS transistor 3 and a word current line 4. A bit current line 5 is placed orthogonal with the word current line 4. When electrical currents flow in the word and bit current lines 4, 5, word and bit magnetic fields 41 and 51 are respectively produced. Electrical currents are typically short current pulses from 2 to 5 nanoseconds having a magnitude on the order of 10 mA. An additional control current line 6 is applied to control the opening and/or the closing of the transistor 3 to address each memory cell 1 individually.
During a writing process, the transistor 3 is in the blocked mode (OFF), and no current flows through the magnetic tunnel junction 2. The intensity of the current pulses and their synchronization are adjusted so that only the magnetization of the storage layer 21 located at the crossing of the word and bit current lines 4, 5 can switch under the combined effect of the word and bit magnetic fields 41 and 51.
During a reading process, the transistor 3 is in the saturated mode (ON) and a junction current will flows through the magnetic tunnel junction 2 allowing the measurement of the junction resistance of the memory cell 1. The state of the memory cell 1 is determined by comparing the measured resistance with the resistance of a reference memory cell. For example, a low junction resistance will be measured when the magnetization of the storage layer 21 is parallel to the magnetization of the reference layer 23 corresponding to a value of “0.” Conversely, a magnetization of the storage layer 21, antiparallel to the magnetization of the reference layer 23, will yield a high junction resistance corresponding to a value of “1.”
Basic structural details for this type of conventional MTJ-based MRAM are described in U.S. Pat. Nos. 4,949,039 and 5,159,513; while, U.S. Pat. No. 5,343,422 discloses an implementation of a random-access memory (RAM) based on a MTJ based MRAM structure.
To help ensure that this architecture is working properly during the writing process, it is necessary to use memory cells 1 with an anisotropic form having high aspect ratios, typically 1.5 or more. Such geometry is required to obtain bi-stable functioning of the memory cell 1, a good writing selectivity between the selected memory cell and the half-selected cells located on the same line/column, and good thermal/temporal stability of the information.
According to U.S. Pat. No. 5,959,880, the aspect ratio of a memory cell can be reduced by increasing the magnetocrystalline anisotropy of the material that forms the storage layer. By doing this, the system is stable in time and temperature, and both states of the memory cell are well defined. On the other hand, the writing field required to reverse the magnetization of the memory cell from one stable state to another is significant and therefore the power consumed during the writing process is large. Conversely, if the magnetocrystalline anisotropy is low, the power consumed at writing is also low, but thermal and temporal stability of the storage layer are no more ensured. In other words, U.S. Pat. No. 5,959,880 teaches that it is not possible to simultaneously ensure low power consumption and thermal and temporal stability.
A thermally assisted writing switching (TAS) process for the above-referenced MTJ-based MRAM structure is described in United States Patent Application Publication No. US 2005/0002228 A1. The particularity of the magnetic tunnel junction of the TAS MTJ based MRAM is that both the reference layer and the storage layer are exchange biased. More precisely, the reference and storage layers are pinned by interaction with an adjacent antiferromagnetic reference layer and antiferromagnetic storage layer respectively. During a thermally assisted writing process, for example, a junction current pulse is sent through the magnetic tunnel junction rising the temperature of the magnetic tunnel junction and the magnetic coupling between the ferromagnetic storage layer and antiferromagnetic storage layer disappears. The magnetic tunnel junction is then cooled while a moderate magnetic field is applied by making a current to flow in the word current line, allowing for the reversal of the magnetization of the storage layer.
In contrast with the conventional MTJ-based MRAM, the TAS MTJ based MRAM structure is characterized by a considerably improved thermal stability of the storage layer due to the pinning of the antiferromagnetic storage layer. An improved writing selectivity is also achieved due to the selective heating of the memory cell to be written in comparison with the neighboring memory cells remaining at ambient temperature. The TAS MTJ-based MRAM structure also allows for a higher integration density without affecting its stability limit, and reduced power consumption during the writing process since the power required to heat the memory cell is less than the one needed to generate magnetization in the conventional MTJ-based MRAM structure.
A further improvement of the TAS MTJ-based MRAMs in terms of power consumption has been described in United States Patent Application Publication No. US 2006/0291276 A1. Here, the writing field is further reduced by selecting a circular geometry of the memory cell junction. In this case, the writing field is only given by the magnetocrystalline anisotropy of the storage layer and there is no contribution from the shape anisotropy. However, the use of a circular geometry does not allow for simultaneously low power consumption and thermal and temporal stability of the storage layer.
The benefit of using a circular magnetic tunnel junction can be better understood by expressing the energy of the magnetic barrier height that has to be overcome to write the cell from a state “0,” of low electrical resistance, to a state “1,” of high electrical resistance. The barrier energy per volume unit, Eb, can be expressed as set forth in Equation 1.
                              E          b                =                  K          +                                                                      A                  ⁢                                                                          ⁢                  R                                -                1                            L                        ⁢                          tM              s              2                                                          (                  Equation          ⁢                                          ⁢          1                )            
In Equation 1, the first term, K, is the magnetocrystalline anisotropy and the second term corresponds to the shape anisotropy. In the second term, AR is the aspect ratio of the magnetic tunnel junction, defined as the ratio of the length to the width L of the magnetic tunnel junction; t is the thickness of the storage layer; and Ms its saturation magnetization. The ellipticity can be defined as (AR−1), expressed in percentage terms.
The limitations of the prior art can be understood by considering that the barrier energy Eb increases with decreasing the size of the magnetic tunnel junction (L decreases and AR is constant), resulting in a significant increase in power consumption. In the other hand, the barrier energy Eb decreases with decreasing AR (L being constant), resulting in a loss of thermal and temporal stability.
In the case of a TAS MTJ-based MRAM with an exchange-biased storage layer, the storage layer stability at working temperatures is ensured by the pinning of the ferromagnetic storage layer with the antiferromagnetic layer, while, at writing temperatures, the pinning disappears and the memory cell can be written with a low writing field. In the case of a circular cell junction, a low writing field is obtained only by the low magnetocrystalline anisotropy. A low writing field and good thermal stability can then be obtained simultaneously by combining the junction geometry with the TAS MTJ-based MRAMs.
However, usual MRAM fabrication processes cannot guarantee perfectly circular magnetic tunnel junctions over a large array of memory cells, due to, for example, accuracy limitations in the pattering of the different junction layers. In addition, the magnitude of writing fields is strongly dependant on variations in the junction ellipticity. FIG. 2 shows the dependence of the writing field HR of the storage layer, on the aspect ratio of the magnetic tunnel junction for a conventional TAS MTJ-based MRAM cell. For example, the magnitude of the writing field more than doubles when the junction aspect ratio is increased from AR=1.0 to 1.1, representing a 10% variation typical from a usual fabrication process. The inset of FIG. 2 shows a top view of magnetic tunnel junctions with aspect ratio comprised between 1.0 and 1.1.
Such a variation of the aspect ratio results in a large dispersion of the writing field and a significant increase of the power consumption in a magnetic memory device containing an array of memory cells with circular junctions. In addition, electromigration effects in the current lines that occur for large electrical currents at high writing field may not be avoided.